1. Field of the Invention
The present invention relates to a sheet feeding device including: a sheet stacking portion on which a multiple number of sheets are loaded; a feed roller for feeding a sheet from the sheet stacking portion; and a frictional plate which is pivotally supported on a pivot axle and urged toward the feed roller in order to prevent a multiple number of sheets from being delivered out by the feed roller, and in particular relates to a sheet feeding device for use in an image forming apparatus such as a copier, printer, printing press, etc. to achieve one-by-one conveyance of sheets, without causing multiple delivery of sheets.
2. Description of the Prior Art
Conventionally, Japanese Patent Application Laid-Open Hei 7 No. 20,620 has disclosed that a sheet conveying device in which a frictional plate is pressed against the feed roller in order to convey sheets one by one without causing multiple delivery of sheet.
This sheet conveying device includes: a sheet stacking portion on which a multiple number of sheets are loaded; a feed roller for feeding a sheet from the sheet stacking portion; and a frictional plate which is pivotally supported on a pivot axle and urged against the feed roller side in order to prevent a multiple number of sheets from being delivered out by the feed roller. In this arrangement, the pivotal axle of the frictional plate was disposed on the upstream side, with respect to the sheet conveying direction, of the pressure nip between the feed roller and the frictional plate and on the side opposite to the feed roller, with respect to the tangent line of the frictional plate to the feed roller.
Even in a configuration where the pivotal axle of the frictional plate was disposed downstream of the pressure nip between the frictional plate and the feed roller, the axle was positioned on the side opposite to the feed roller, with respect to the tangent line of the frictional plate to the feed roller.
Japanese Patent Application Laid-Open Hei 8 No. 40,577 disclosed a configuration in which the feed roller is composed of two parts, i.e., D-shaped roller portion for sheet feeding and idle roller portion which is freely rotatable with separate frictional plates provided in pressure contact with all of them, whereby multiple delivery of sheets was prevented and handling the performance of sheet jamming was improved, during the respective modes, i.e., feeding mode and non-driven mode.
In the aforementioned sheet conveying device, as shown in FIGS. 1A and 1B, a pivotal axle 51a of a frictional plate 51 is disposed on the upstream side of the contact between a feed roller 52 and frictional plate 51 and on the side opposite to feed roller 52, with respect to the tangent line of feed roller 52 and frictional plate 51. In this case, a frictional force F of load acting on frictional plate 51, along the tangent at the pressure nip between feed roller 52 and frictional plate 51, during the rotation of feed roller 52, can be decomposed into a first force component F1 in the radial direction from pivotal axle 51a of frictional plate 51 to the nip and a second force component F2 in the tangent direction of rotation of frictional plate 51 at the nip. Force component F2 in the tangent direction of rotation of frictional plate 51 acts so as to move the frictional plate away from feed roller 52; this tendency is unpreferred in view of anti-multiple delivery of sheets and is preferred in view of reducing the passively driven load after the sheet is nipped by the rollers on the downstream side.
Consider the cases where sheets are pulled out in the reverse direction to the sheet feeding direction of the feed roller, for canceling jamming or for changing the sheets. In this case, a frictional force G of load acting on frictional plate 51, along the tangent at the pressure nip between feed roller 52 and frictional plate 51, during the rotation of feed roller 52, can be decomposed into a first force component G1 in the radial direction from pivotal axle 51a of frictional plate 51 to the nip and a second force component G2 in the tangent direction of rotation of frictional plate 51 at the nip. Force component G2 in the tangent direction of rotation of frictional plate 51 acts so as to press the frictional plate against the feed roller, causing difficulty in pulling out the sheets.
This tendency becomes more critical as pivotal axle 51a is positioned more distant from the tangent line.
On the contrary, when this pivotal axle 51a is positioned on the downstream side, with respect to the sheet conveying direction, of the pressure nip between feed roller 52 and frictional plate 51 and on the side opposite to the feed roller, with respect to the tangent line of frictional plate 51 to feed roller 52, the situation becomes completely opposite to the above description.
In the former configuration, an extra mechanism for releasing the pressure of frictional plate 51 is needed. In the latter case, image defects such as image magnification failure or pitch irregularity due to load variation during image forming process on a sheet are liable to occur during the image forming process.
In the case of Japanese Patent Application Laid-Open Hei 8 No. 40,577, the feed roller is composed of two parts, i.e., the D-shaped roller portion for sheet feeding and idle roller portion which can freely rotate, with separate frictional plates provided for all of them. Even with the use of the idle rollers, it cannot but receive some load because of the pressure of the frictional plates against these idle rollers. As a result, some image defects such as image magnification failure or pitch irregularity occur during the subsequent image forming process, due to variation of load.
In recent years, in order to make the appliances more compact and lower in cost, many image forming apparatus often have adopted the configuration in which the sheet is fed directly from the feed roller to the image forming unit, instead of using an intermediate conveying roller. As a result, the above problem has become of more importance and cannot be neglected.